The invention concerns hydrostatic drives with a closed hydraulic circuit consisting of at least one hydraulic motor and one variable displacement pump. In particular, the invention concerns a control device for the controlled pressurization of a servo control unit for a variable displacement pump within a hydraulic circuit. The servo control unit of the variable displacement pump is preferably double-sided, while the variable displacement pump is preferably reversible, though this is not necessary for the inventive concept to be constituted, since the inventive concept can also be realized with a hydraulic pump that is adjustable on one side. The inventive concept applies to all types of hydraulic motors and pumps, in particular axial piston and radial piston types. This includes fixed displacement motors, variable displacement motors and reversible motors of the axial piston or radial piston type, though the inventive concept can be applied to all machines which operate hydrostatically. More broadly speaking, the invention also includes hydrostatic linear drives insofar as these are integrated in a closed hydraulic circuit.
Hydrostatic drives with a closed hydraulic circuit are known from the prior art. They are used for many types of drive systems in mobile and statically positioned machines. For example they are used as propulsion power units for vehicles and/or as ancillary drive units such as fans, generators, cooling units and similar. The inventive concept encompasses all hydraulic drives which comprise at least one hydraulic pump adjustable by servo control and operating within a closed circuit.
Technology always tends towards constantly increasing performance, often combined with the requirement to provide a facility to change the direction of rotation or the direction of the drive. At the same time, the aspect of adapting the performance to actual requirements is becoming increasingly important from the point of view of saving energy. Hydraulic drives are frequently used due to the high levels of efficiency they are capable of achieving and due to their robust and reliable power transmission. Hydraulic drives are also given preference when the area of application involves large fluctuations in temperature and/or large fluctuations are to be expected in power consumption and power delivery. For years, the regulation and control mechanisms and the devices used in this context have proven to be very flexible, fast and highly reliable.
In a closed hydraulic circuit, hydraulic pumps are typically used which demonstrate the same characteristic interrelation between input signal and displacement volume. This is conventionally accomplished by means of servo control devices actuated by control devices which adjust the hydraulic unit by means of deflection elements such as swashplates or bent axes. Here it is usual for the displacement volume of the hydraulic machine to be proportional to the input signal. Adjustable and/or reversible hydraulic drive units with a closed hydraulic driving circuit are used as propulsion power units, for example, with the possibility of moving in both directions. If the input signal fails, the supply pumps are often placed in a zero position in which they do not exhibit any displacement volume, thereby bringing a propulsion power unit to a standstill, for example.
Essentially, servo control devices with a zero position can also be used for drives in which the servo control device is not placed in a zero position if the input signal fails but in a displaced position so that in the case of a fan drive, for example, the fan is still capable of cooling. This is an emergency function, in particular, in the case of electrical failure since control devices for hydraulic drives are often actuated by means of electric proportional magnets
A special control device for fan drives in a hydraulic circuit is described in DE 10 2010 009 975 A1. The control device described there ensures, for example, that a fan works at maximum rotational speed if the input signal at the control device fails. In this case, the control piston in the control device, which is guided in a double-sided control cylinder, is shifted into a maximum deflected position against a mechanical fixed stop by means of a spring in case of input signal failure. When it is shifted into a maximum displaced position in the control cylinder, the control piston opens a feed line to a servo control of a variable displacement pump, thereby opening a discharge line for drive fluid on the other side of the control cylinder. The result is that the servo control device is supplied with pressurized drive fluid on one side. In the example selected of a control piston pressurized from one side, the latter and the servo piston hydraulically connected to the control device are deflected to the maximum extent and the fan is powered by an hydraulic motor at maximum drive power. According to the state of the art, a zero-stroke valve limits the hydraulic pressure applied to the hydraulic motor, thereby limiting the maximum rotational speed of the fan.
A proportional magnet can be used to apply an input signal to the control piston in the form of a magnetic force, with the result that the control piston is able to counteract the deflecting force of the spring as the input signal increases, i.e. as the magnetic force increases, thereby shifting the control piston towards neutral position or even to the other side in the control cylinder. When the control piston in the control cylinder is in neutral position, the feed and discharge lines to the servo control device are virtually at equalized pressure. This puts the servo control in a zero position and the hydraulic pump is not deflected, so it does not exhibit a flow rate. If there is no hydraulic flow in the hydraulic pump, the hydraulic motor cannot be powered, which is why the fan logically comes to a standstill.
If the control piston is shifted beyond neutral position by the force of the proportional magnet to the other, second side in the control cylinder, a feed line for drive fluid is opened on the second side of the control cylinder to the servo control device, while at the same time on the first side of the control cylinder, on which the feed line was previously open, a discharge line for drive fluid is opened so that the servo control device can deflect the hydraulic pump to the other side and the direction of flow of the hydraulic fluid in the closed circuit is reversed. Assuming the concept of a fixed displacement motor or a hydraulic motor which is adjustable with variable flow capacity on one side only, the rotational direction of the motor is thus altered, with the result that the rotational direction of the fan can also be reversed. In fans for internal combustion engines in particular, one rotational direction is referred to as the cooling effect of the fan and the reversed rotational direction is referred to as the aeration capacity, by means of which warm air can be blown out of the vehicle or machine into the environment and/or the fan is cleaned, for example.
For it to function as planned, the system described above according to DE 10 2010 009 975 A1 requires pressure relief valves, preferably in the form of zero-stroke valves, which limit the maximum pressure on the respective side in the servo system so that the hydrostatic system is not overloaded. This configuration of pressure relief valves to limit maximum capacity in the feed and discharge lines of the servo device is complex in design and expensive to construct. What is more, pressure relief valves or zero-stroke valves such as these occupy a large amount of installation space in the housing of the variable displacement pump, making it necessary to create a large housing volume. By limiting maximum capacity via pressure relief valves in the servo pressure lines or by means of zero-stroke valves which limit the maximum deflection of the servo control device via the operating pressure (cf. DE 10 2010 009 975 A1), the power supplied to the hydraulic motor of the hydrostatic drive is dependent on the rotational speed of the supply pump. The higher the rotational speed of the supply pump, the higher the displacement volume and supply pressure to be limited by the zero-stroke valves. However, as long as the supply pressure remains below a set limit at which the zero-stroke valves respond, fluctuations in pump output are passed on to the hydraulic motor.